Flow cytometry was used to investigate the effects of amiloride on cell cycle progression, and the effectiveness of amiloride to attenuate the S and G2 phase checkpoint responses induced by 2.5, 5.0 and 7.5 Gy of gamma radiation. The late S phase delay and the radiation-induced G2 block were both significantly reduced in amiloride-treated samples without apparent apoptosis or necrosis over a 24 h period. Results indicate that amiloride reduces the radiation-induced G2 block in HL-60 cell populations almost equally well as caffeine, and to a greater extent than staurosporine. Alterations in intracellular ion concentrations induced by amiloride may lead to changes in Ca2+-dependent signaling cascades, and thereby decrease the radiation-mediated cell cycle perturbations. Future studies are designed to determine the effects of amiloride on the expression of various cyclins, particularly cyclins A and B1.